To understand the mechanism by which a conditioned reflex is formed, the conditioned stimulus may be traced from the periphery. At one point, the afferent signal must be processed differently after learning to produce the new behavioral response. Changes at this point are the first detectable ones due to learning. More peripherally, the signal is treated similarly before and after learning. Cochlear nucleus may be the locus of the initial locus of change on the auditory pathway. Eye blink conditioning in rabbit will be used as the behavioral preparation. Processing of the auditory conditioned stimulus during learning will be studied physiologically in the cochlear nucleus by extracellular recording. Single neurons which have been recording will then be labeled from the extracellular space with the enzyme horseradish peroxidase and visualized post-experimentally. The bushy and fusiform cells types will be analyzed during the conditioning process. From their physiological and anatomical profiles, it is hypothesized that the bushy cells will remain constant while the fusiform cells will change. Both cell types receive direct afferents from the auditory nerve. The relation of plasticity with position in the tonotopic array will be of particular interest. Data gathered in the present work will be used as a basis for future research in more central regions. The developing auditory reflex arc will be examined. It is hoped that learning centers may be characterized physiologically and anatomically.